1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus having the zoom lens, and the zoom lens is suitably used for an image pickup apparatus using a solid-state image pickup element or an image pickup apparatus such as a camera using a silver-salt film.
2. Description of the Related Art
As an image pickup optical system that is used for an image pickup apparatus, a small-size zoom lens having a short total lens length with a wide angle of field and a high zoom ratio is required. A high-resolution zoom lens in which a chromatic aberration, as well as a monochromatic aberration such as a spherical aberration or a coma aberration, is appropriately corrected is also required.
A positive-lead type zoom lens that includes, in order from an object side to an image side, first, second, and third lens units having positive, negative, and positive, respectively, and a rear lens group including at least one lens unit is known.
With respect to a positive-lead type zoom lens having four units, the zoom lens for which an anomalous dispersion material is used for a material of positive lenses of a first lens unit and a third lens unit and a chromatic aberration is appropriately corrected is known (Japanese Patent Laid-Open No. 2010-91788).
With respect to a zoom lens having five units configured by lens units having positive, negative, positive, negative, and positive refractive powers in order from the object side, the zoom lens that uses a lens configured by a material having an anomalous dispersion as the third lens unit is known (Japanese Patent Laid-Open No. 2010-32700).
In addition, a zoom lens with a wide angle of field and a high zoom ratio which is configured by five lens units having positive, negative, positive, positive, and positive refractive powers in order from the object side to the image side is known (Japanese Patent Laid-Open No. 2004-117826).
It is relatively easy for the positive-lead type zoom lens to reduce a size of a total system and achieve the wide angle of field and the high zoom ratio. However, in most of the positive-lead type zoom lenses, a lot of secondary spectra of an axial chromatic aberration are generated in a zoom area at the telephoto end when a focal length at the telephoto end is increased so as to achieve the high zoom ratio.
In order to reduce the chromatic aberration, it is effective to use a lens configured by a material with a low dispersion and an anomalous dispersion at an appropriate position in the zoom lens. In addition, it is important to optimize each lens unit constituting the zoom lens based on characteristics of materials (Abbe's number or a partial dispersion ratio).
In particular, in a positive-lead type zoom lens configured by four units or five units, it is important to appropriately set a refractive power or a lens configuration of the first lens unit or the third lens unit. In addition, it is important to appropriately set a moving amount of the first lens unit or the third lens unit in zooming.
For example, when a long focal length where a focal length at the telephoto end is over 1000 mm for a converted focal length by the zoom lens using a silver-salt film is included, it is difficult to reduce the axial chromatic aberration only by using a glass material having the anomalous dispersion as the first lens unit.
In order to reduce the axial chromatic aberration at the telephoto end, it is preferred a glass material with the anomalous dispersion is used as each of positive lenses of the first lens unit and the third lens unit. However, when the glass material is used as each of the positive lenses of the first lens unit and the third lens unit, a chromatic aberration of magnification is increased at the wide-angle end.